1. Field of the Invention
The present invention is generally directed to a current mirroring circuit. More specifically, the present invention is directed to a circuit and method for mirroring currents in application specific integrated circuits (ASICs) with an accuracy greater than previously obtainable using matched active devices.
2. Background
Various techniques are used to provide regulated current to a load circuit. One such technique involves a current mirroring circuit which is used to provide an output current equal to or proportional to a reference input current. Current mirroring is typically accomplished by putting current through an active reference device such as a bipolar or MOS device. The resulting gate to source voltage in a field-effect transistor (FET) for example, can then be applied to multiple other devices which closely match and are connected to the original reference device. Current which mirrors the reference current then flows through each of the other multiple devices to the extent these devices are identical. A total mirrored current which is larger than the reference current by the same number of times as there are multiple devices connected to the reference device is achieved by combining the device outputs.
An important aspect in the design of current mirroring circuits is achieving an optimum match between the input reference current and the output current of each mirroring device. Since this current mirroring accuracy assumes that the active mirroring devices are fabricated with similar traits, current mirroring circuits are commonly fabricated on monolithic substrates as part of an integrated circuit such as an ASIC. Additionally, nominally equal value resistors are sometimes added in series with the source of each active device to improve the mirroring accuracy. This technique is successful to the extent that the matching of the resistors is better that the matching of the active mirror devices.
Nevertheless, prior art methods of achieving current mirroring accuracy in ASICs or other monolithic integrated circuits continue to suffer from problems in matching active mirror devices, especially where mirroring ratios beyond two are desired. Where higher mirroring ratios are desired, the associated increase in the number of mirror devices precludes placing all the mirror devices directly adjacent to the reference device. Therefore, device mismatch is increased by the gradients of dimensional accuracy and doping as the mirror devices are spread across the chip. The result is a reduction in current mirroring accuracy when higher mirroring ratios are desired because of the necessary reliance on matching the active mirror devices.
Accordingly, there exists the need for a method of mirroring currents in application specific integrated circuits with an accuracy better than obtainable by mirroring with matched active devices.